The present invention relates to an improvement in the latching of one or more pawls within a pawl and ratchet mechanism. A pawl and ratchet mechanism with the improved latching mechanism of the present invention may be subjected to and successfully contain explosive forces. When forces of great magnitude are suddenly applied to pawl and ratchet mechanisms that exist in the prior art, such explosive forces may cause a pawl to fail to latch within the ratchet. Such failure is due to the fact that an explosive force may cause a pawl to be thrown upward out of latching engagement with the ratchet. The speed with which a disengaged pawl is thrown back by an explosive force can prevent the pawl from engaging any other portions of the ratchet.
While the improved latching mechanism of the present invention may find application in any situation in which a pawl and ratchet mechanism is employed, the application and embodiment of the present invention described herein is designed for use in hydraulic testing of tubular members such as drill pipe and casing used in the oil and gas producing industry.
Hydraulic pipe testing apparatus is well known in the prior art. Generally speaking, such apparatus comprises means for sealing the ends of a pipe to be tested and means for introducing and removing hydraulic fluid from the pipe through the apparatus used to seal the ends of the pipe. The means for sealing each pipe end usually comprises a threaded plug which is rotatably mounted on a wheeled carriage adapted to roll on tracks parallel to the length of the pipe.
In the pipe testing operation, the pipe is first placed upon and secured onto a pipe supporting stand. Two of the aforementioned wheeled carriages are rolled up to the ends of the pipe to be tested so that the rotatably mounted threaded plugs carried by the carriages are brought into alignment with the ends of the pipe. The threaded plugs are then inserted into the ends of the pipe and rotated to threadably engage and seal the ends of the pipe.
In order to prevent the movement of the carriages away from the pipe during pipe testing operations, numerous types of pipe testing apparatus in the prior art have utilized pawl and ratchet mechanisms. Such mechanisms usually involve the use of two toothed tracks aligned parallel to the direction of motion of the carriage. A pivotally movable arm or bar is usually mounted on the carriage to serve as a pawl for engaging the ratchet formed by the teeth of the toothed tracks. Once the carriage is in place and the threaded plug has sealed the end of the pipe, the pivotally movable arm or bar is mechanically or hydraulically lowered so that its non-pivoted end comes to rest in a space between the teeth of one or more of the toothed tracks, thus preventing movement of the carriage away from the pipe.
The pawl and ratchet mechanisms disclosed in the prior art are not designed to restrain a carriage which is subjected to the explosive forces observed during high pressure hydraulic failure occurring during the testing of large diameter pipe such as casing. Because the hydraulic fluid used to internally test casing must be under high pressure and because the cross-sectional area of casing is large, the explosive force with which the threaded plug is thrown from the pipe during failure may be very great. A comparison of the magnitude of the forces involved in hydraulic testing is illustrative. A typical test pressure for small two-inch (2") inner diameter tubing is about 7,000 p.s.i. This pressure applied to the circular cross-sectional area of the tubing (3.14 square inches) yields a force of approximately 22,000 lbs. acting upon the threaded connection of the threaded plug. The inner diameter of casing, however, is on the order of thirteen inches (13") and the test pressures are typically in the 3,000 p.s.i. range. Such pressure acting upon the circular cross-sectional area of the casing (133 square inches) yields a force of approximately 400,000 lbs. acting upon the threaded connection of the threaded plug. Such force can easily cause the threaded plug to be blown out of the end of the pipe if the plug has been improperly threaded into the pipe or if a structural failure of the threads of the pipe or of the threads of the plug occurs.
One approach to solving the problem of containing a plug thrown from a pipe during high pressure hydraulic testing has been to provide a large massive wall or receptacle into which the plug may be thrown without damage. The primary drawback to this approach is that it is not adaptable for use on portable pipe testing apparatus. The size and weight of a large massive retaining wall or receptacle dictates that such method is practical only for stationary pipe testing apparatus.
The present invention finds application in providing means for containing the explosive force occurring during a pressure failure of the type described wherein said means are adapted to be used with a portable pipe testing apparatus.